The present invention relates to a perpendicular magnetic recording medium and a magnetic storage apparatus capable of recording a large amount of information.
According to a longitudinal magnetic recording system adapted to existing magnetic disk units, for improvement of a linear recording density, it is necessary to reduce a demagnetizing field in a recorded bit by decreasing the product of a remanent magnetization of a magnetic film as a recording medium and the thickness of the magnetic film. Moreover, the coercivity of the magnetic film must be increased. However, as the coercivity of a magnetic recording layer is increased, the ability of a recording head to write data becomes insufficient. Moreover, for reduction in a medium noise, it is necessary to reduce the sizes of crystal grains and the grain size distribution. However, such a medium having the sizes of ferromagnetic crystal grains thereof reduced poses a problem in that recorded information may be lost due to thermal fluctuation. This makes it hard to improve the recording density according to the longitudinal magnetic recording system.
For solution of the foregoing problems, a perpendicular magnetic recording system is widely recognized. The perpendicular magnetic recording system is a system of forming recorded bits so that a recording medium will be magnetized perpendicularly to the surface of the recording medium and the magnetization directions in adjacent recorded bits will be anti-parallel to each other. Compared with the longitudinal recording system, a demagnetizing field at a magnetic transition region is small. Therefore, a medium noise can be reduced, and at high recording densities a recorded magnetization can be held stable.
As a magnetic recording layer (magnetic layer) included in a perpendicular magnetic recording medium, adoption of a Co—Cr—Pt alloy film that is adapted to longitudinal magnetic recording media, or adoption of a multilayer film having numerous Co and Pt layers alternately formed has been discussed. For reduction in a medium noise caused by the medium whose recording layer composed of the Co—Cr—Pt alloy film or multilayer film, and for improvement in thermal decay of recorded magnetization, it is important to reduce the crystal grain sizes of the recording layer, reduce the grain size distribution, and reduce the intergranular exchange coupling.
When the Co—Cr—Pt alloy film is adopted as a magnetic recording layer, the c-axes of the hexagonal close-packed (hcp) structures are aligned perpendicularly to the surface of the film. The longitudinal crystal orientation is only a little different between adjacent crystal grains. Therefore, the segregation of Cr to grain boundaries hardly takes place. This results in insufficient magnetic decoupling of crystal grains or an increase in crystal grain size due to coalescence of crystal grains in the course of crystal growth. Consequently, it is hard to reduce medium noise.
In efforts to solve the above problem, a proposal has been made of a magnetic recording medium structured so that ferromagnetic crystal grains will be enclosed with non-magnetic compounds such as oxides or nitrides. The thus-structured magnetic recording medium (or magnetic recording layer) is referred to as a granular medium (or granular magnetic recording layer). For example, Japanese Unexamined Patent Application Publication No. 2002-342908 has disclosed a medium having Si oxides, which contain silicon by an atomic percent equal to or larger than 8 and equal to or smaller than 16, added to a Co—Cr—Pt alloy. When a large quantity of oxides is added, the c-axis orientation of crystal grains in a magnetic recording layer may be degraded or the oxides may be mixed not only in grain boundaries but also in ferromagnetic crystal grains. This poses a problem in that a coercivity, a squareness ratio, a signal-to-noise ratio, and a resolution decrease.
In order to solve the above problem, it is supposedly important to control the crystal growth of a magnetic recording layer using an underlayer or an intermediate layer. For example, “High Performance CoPtCrO Single Layered Perpendicular Media with No Recording Demagnetization” (IEEE Transactions on Magnetics, Vol. 36, No. 5, September 2000, pp. 2393-2395) has disclosed a single-layer perpendicular medium having a CoPtCrO magnetic layer formed on an Ru underlayer. The thickness of an Ru intermediate layer is equal to or larger than about 40 nm, whereby the c-axis orientation of ferromagnetic grains improves. Consequently, the magnetic properties and recording performances of the magnetic recording layer improve.
Moreover, Japanese Unexamined Patent Application Publication No. 6-76260 has disclosed a structure that has an intermediate layer, which is composed of a metal or an alloy having the face-centered cubic (fcc) lattice structure, formed on a Ti underlayer, and that has a CoPtBO granular magnetic recording layer formed on the intermediate layer. Since the intermediate layer made of a metal or an alloy having the fcc lattice structure is formed on the Ti underlayer, the structure exhibits high magnetic anisotropy within a wide range of concentrations of oxygen to be added to the CoPtBo granular magnetic layer. This results in an improved squareness ratio. Nevertheless, the squareness ratio is smaller than 0.5 and insufficient. Furthermore, the intermediate layer composed of a metal or an alloy having the fcc lattice structure is as thick as 100 nm. If the structure is adapted to a double-layer perpendicular medium, there is a fear of degradations in write-ability and a recording resolution.
In a double-layer perpendicular magnetic recording medium having a soft-magnetic underlayer formed under a magnetic recording layer, a non-magnetic layer (intermediate layer) interposed between the magnetic recording layer and soft-magnetic layer causes a spacing loss during recording. In order to prevent degradations of write-ability and a recording resolution, the intermediate layer for controlling the crystal growth of the magnetic recording layer must be thinned.
As an example of a magnetic recording medium having a thin non-magnetic intermediate layer, Japanese Unexamined Patent Application Publication No. 2003-77122 has disclosed a structure comprising a CoCrPt—SiO2 granular magnetic layer, a non-magnetic underlayer that is composed of a metal or an alloy having the hexagonal close-packed (hcp) structure and that is formed under the magnetic layer, a seed layer that is composed of a metal or an alloy having the face-centered cubic (fcc) lattice structure and that is formed under the non-magnetic underlayer, and a non-magnetic orientation control layer that is composed of a metal or an alloy having the body-centered cubit (bcc) lattice structure or the amorphous structure and that is formed under the seed layer. The (0002) planes of the non-magnetic underlayer having the hcp structure are epitaxially grown on the seed layer with the fcc (111) texture, whereby the crystallographic texture is improved. Consequently, excellent magnetic properties are attained despite a thin intermediate layer whose thickness is equal to or smaller than 20 nm. In this case, the thickness of the seed layer composed of the metal having the fcc lattice structure must be equal to or larger than 3 nm, or more preferably, equal to or larger than 5 nm for the purpose of improving the crystallographic texture. Moreover, Japanese Unexamined Patent Application Publication No. 2003-178412 describes that when the non-magnetic underlayer included in the above structure is composed of a Ru-based alloy capable of establishing good lattice matching with the magnetic recording layer by adding at least one material of C, Cu, W, Mo, Cr, Ir, Pt, Re, Rh, Ta, or V to Ru, even if the thickness of the intermediate layer is equal to or smaller than 20 nm, the crystal grain sizes of the magnetic recording layer can be reduced and its initial growth layer can be reduced. Consequently, medium noise can be reduced.
Japanese Unexamined Patent Application Publication No. 2002-334424 has disclosed a Co—Cr—Pt magnetic layer, an orientation control layer that has a thickness of 5 nm, that is composed of Ru to which contains 20 atomic percent of oxides containing such as Si, Zr, Hf, Ti, or Al, or 15 atomic percent of B or C is added, and that is intended to control the orientation of the magnetic layer, and an orientation control underlayer composed of NiAl having the B2 structure and intended to control the orientation of the orientation control layer. When such a large quantity of oxides, B, or C is added to Ru, although it is intended to improve the orientation of a recording layer and reduce the grain size thereof, the crystallographic texture may be degraded. In reality, “Medium Noise and Grain Size Analysis of CoCrPt/Ti Perpendicular Media with NiAl Seed Layer” (IEEE Transactions on Magnetics, Vol. 37, No. 4, July 2001, pp. 1583-1585) reports that layering Ti, which has the hcp structure, on NiAl having the B2 structure has the merit of reducing a grain size but poses a problem in terms of the c-axis orientation.
According to the foregoing related arts, the magnetic isolation of ferromagnetic crystal grains constituting a magnetic recording layer is insufficiently attained or the crystallographic texture is unsatisfactory. For realization of a perpendicular magnetic recoding medium capable of achieving higher-density recording, it is necessary to devise a technology for producing an intermediate layer that promotes the magnetic isolation of ferromagnetic crystal grains from one another while maintaining the crystal crystallographic texture of a magnetic recording layer and thus helps reduce its noise.
When a multilayer film having Co and Pd layered is adopted as a magnetic recording layer, compared with when a Co—Cr—Pt alloy is adopted, intergranular exchange coupling of the magnetic recording layer is quite large. For high-density recording, the crystal grains constituting the multilayer film must be magnetically isolated from one another.
For example, Japanese Unexamined Patent Application Publication No. 2002-25032 has disclosed that B and O are contained in a recording layer in order to magnetically isolate crystal grains, which constitute a magnetic recording layer, from one another. Moreover, Japanese Unexamined Patent Application Publication No. 2002-304715 has disclosed the following: N and O are contained in a magnetic recording layer consisting of a multilayer film and an underlayer containing at least Pd and Si is employed, whereby crystal growth in the multilayered structure and reduction in intergranular exchange coupling are realized. Discussion of an intermediate layer and a seed layer adaptable to the multilayer film is made in “Perpendicular magnetic recording thin film media using Co/Pd superlattice on ultrathin indium-tin-oxide seed layers” (Journal of Applied Physics, Vol. 87, No. 9, May 2000, pp. 6358-6560) that has disclosed a structure that Pd is formed on an indium-tin-oxide (ITO) layer. Moreover, “Co/Pd multilayer media with Pd inorganic granular seed layer for perpendicular recording” (Journal of Applied Physics, Vol. 91, No. 10, May 2002, pp. 8073-8075) has disclosed a structure using PdSiN. In a Co/Pd multilayer medium, perpendicular magnetic anisotropy is attributed to the structure of the interface between a Co layer and a Pd layer. In case of the Co/Pd multilayer medium, the magnetic properties are less sensitive to the crystallographic texture than in the case of a CoCr-based alloy medium thereof. The related arts have put emphasis on reducing intergranular exchange coupling of a magnetic recording layer, and are therefore not satisfactory in terms of crystallographic texture. However, the magnitude of magnetic anisotropy of a multilayer medium varies depending on crystallographic texture. Improving the crystallographic texture for reducing a distribution of the magnitude of magnetic anisotropy is presumably important for improvement of thermal stability and recording performance.